As an image forming apparatus referred to as MFP (Multi-Functional Peripheral), a full-color type having plural photoconductive drums is well known.
The full-color MFP superimposes images held by the respective photoconductive drums. Therefore, “inconsistency of overlap of the images” called color drift occurs.
There is known a technique for, in order to reduce color drift, forming test pattern images in single-color image forming units including photoconductive drums, detecting the test pattern images on an image bearing member such as a transfer belt (used for superimposing images) using a sensor for alignment control, and correcting starting positions or the like for drawing images on respective photoconductive members in the respective single-color image forming units.
For example, JP-A-2008-76546 discloses that an image forming apparatus including plural photoconductive members performs phase control for adjusting angular velocities of respective photoconductive members to prevent misalignment in image formation due to a difference in speed of the respective photoconductive members.
In JP-A-2008-76546, as phase control, the rotation of a motor is controlled while the photoconductive members make full rotation.
However, in an MFP normally including four photoconductive members and an image bearing member such as a transfer belt, it is extremely difficult to control, while the photoconductive members make full rotation, the rotation (angular velocities) of the photoconductive members to suppress fluctuation in a rotation period.